Images produced using imaging devices may contain anomalies that are not representative of the image that was intended to be captured by the imaging device. That is, due to various reasons, images may include features that are not present in the real world image.
Causes of the anomalies may include, for example: contaminants in an imaging device such as dust that has settled on a lens, mirror or other equipment; irregularities in a mirror, lens or other equipment used in an imaging device; and/or image processing errors associate with the algorithms used in image production. The anomalies may reoccur in successive images produced using an imaging device.
For the avoidance of doubt it is noted that the term “anomaly” as used herein encompasses anomalies that appear in an image that are imparted onto the image by the imaging device or system. In particular, anomalies may refer to anomalies due to dust settling within an imaging device, such as on a mirror or lens.
The remainder of this document refers to anomalies caused by contaminants. However the skilled person will be aware that the scope of the invention is intentionally intended to cover other anomalies as described above.
Contaminants such as dust are a particular problem in the field of retinal imaging scanners. The remainder of this document refers predominantly to retinal imaging scanners. However, the skilled person will understand that other imaging devices such as, for example, cameras, optical scanners scanning laser ophthalmoscopes, and the like, may also suffer from the same identified problem and that the method of the present invention may also be used to identify anomalies in images produced by these devices.
The appearance contaminants in a retinal image can be manifest in a form similar to pathology such as micro-aneurisms that the retinal imaging scanner may be attempting to identify. It is therefore desirable to maintain the optical surfaces of a retinal imaging scanner free of dust and other contaminants in order to avoid any ambiguity in the classification of certain image features and to maintain the general quality perception of images produced by the scanner.
Current known methods for maintaining optical surfaces free from contaminants include periodic maintenance whereby a retinal imaging device is cleaned by a trained engineer at regular intervals. In some cases the cleaning process is conducted too early and the optical surfaces may not have been in need of cleaning. In other cases the cleaning process is conducted too late and the optical surfaces may have been operating under sub-optimal conditions for a period of time.
Another known method for detecting dust is disclosed in US patent application 2005/0068446, which uses multiple images produced using a digital camera to generate a dust map. The dust map is compared to a history of previous dust maps generated from previous images. The method used to identify dust regions is based on searching for regions of common gradient variations. This approach is likely to be sensitive to noise. Additionally, it is not applicable in cases where spatial motion between images occurs, i.e. the method could not be used when the spatial position of an individual dust candidate is different between images.